Amplifiers are used in a lot of different circuit designs such as in audio or analog circuits. One problem with some amplifiers such as operational amplifiers is that the amplifier's gain may not be linear over its operating amplitude range. This presents a problem for the circuit designer when using the non-linear amplifier in a system design since it is not easy to characterize the amplifier and is especially problematic when designing circuits such as test system circuits which require high levels of accuracy from the amplifier circuits.
A typical non-inverting feedback operational amplifier configuration is shown in FIG. 1. When the open-loop gain of the amplifier is very high, the voltage gain (G) of a voltage feedback amplifier is very closely expressed by the formula: G=1+Rf/Rg, wherein Rf is the resistance of resistor Rf, and Rg is the resistance of resistor Rg. The operational amplifier includes two differential input ports and an output port. The gain of an operational amplifier is generally set by Rf and Rg under the assumption that the open-loop gain of the amplifier is much higher (>10×) and that the input bias current is quite small, which is typical of a voltage feedback amplifier. Thus the open-loop gain of the amplifier would typically not affect the overall gain of the circuit. But if the open-loop gain of the amplifier is <10× of the programmed gain and the gain varies, then it could impact the overall circuit gain. A more likely scenario is the impact of the input bias current. In particular, a current feedback amplifier has significant input bias current on the inverting input, which can effectively put a non-linear resistive load on the inverting input. A current feedback amplifier may have a different formula as shown above due to the input impedance of the amplifier's inverting input.
For the sake of simplicity, let's assume FIG. 1 shows a voltage feedback amplifier. Thus, the gain for the amplifier in FIG. 1 would be 8 which is calculated by the using the above formula for gain G=(I+140/20). If we assume that this amplifier has a gain error because it is, in reality, non-linear, FIG. 2 shows a plot of Vout error as a function of the expected output voltage when going from −10V to +10V for the amplifier shown in FIG. 1. The error is quite small from −2V to +2V, but grows until the error is approximately 5% at −10V and +10V. This Vout or non-linear error presents a problem for a designer using this amplifier since the designer has to take into account the error or risk multiplying the error as a signal moves through a system. In view of the above, a need exists for a simple circuit for linearizing a non-linear amplifier and method for linearizing an amplifier in order to address one or more of the problems addressed above.